1. Field of the Invention
This invention relates to an input circuit of a television tuner for receiving television signals of a VHF band or television signals of a UHF band.
2. Description of the Related Art
FIG. 5 shows a known input circuit of a television tuner (hereinafter called merely “input circuit”). A VHF tuning circuit 42 is connected to an input terminal 41 to which television signals of VHF and UHF bands are inputted. The VHF tuning circuit 42 is a band-switching type tuning circuit that is designed for tuning with each channel of a low or high range of the VHF band. To achieve this construction, the tuning circuit includes four inductance devices 42a to 42d connected in series, a tuning varactor diode 42e connected in parallel with a series circuit of all the inductance devices and a switch diode 42f connected in parallel with the two inductance devices 42b and 42c at an intermediate part of the inductance devices 42a to 42d. 
A junction between the two inductance devices 42b and 42c at another intermediate part of the four inductance devices 42a to 42d is connected to an input terminal 41. A voltage B is applied to an anode of the switch diode 42f through the inductance devices 42a to 42c and its cathode is connected to a collector of a switch transistor 43. An emitter is grounded. A high- or low-level switch voltage Vs is applied to a base of the switch transistor 43. An anode of the tuning varactor diode 42e is grounded, and a tuning voltage Vt is applied to its cathode. A cathode of a coupling varactor diode 44 is connected to a cathode of the tuning varactor diode 42e. An anode of this varactor diode 44 is grounded by a bias resistor 45 and is connected to a VHF high frequency amplifier 46.
A UHF tuning circuit 47 is connected to the input terminal 41. The UHF tuning circuit 47 tunes with channels of a UHF band and includes an inductance device 47a and two tuning varactor diodes 47b and 47c the cathodes of which are connected to each other and which are connected in parallel with the inductance device 47a. The tuning voltage Vt is applied to the cathodes. The cathodes are connected also to a UHF high frequency amplifier 48.
To receive television signals of a low range (VHF/Lo) or a high range (VHF/Hi) of the VHF band or of the UHF band (UHF) in the construction described above, the switch diode 42f (D1) is turned ON or OFF as shown in FIG. 6. To turn OFF the switch diode 42f, a switch voltage of a low level is applied to the base of the switch transistor 43. To turn it ON, a switch voltage Vs of a high level is applied. When the television signals of the VHF band are received, the VHF high frequency amplifier 46 operates but the UFH high frequency amplifier 48 does not operate. When the television signals of the UHF band are received, on the other hand, the VHF high frequency amplifier 46 does not operate but the UHF high frequency amplifier 48 operates.
Therefore, when the television signals of the low range of the VHF band are received, the switch diode 42f is turned OFF, and the inductance devices 42a to 42d and the tuning varactor diode 42e set the tuning frequency. When the television signals of the high range are received, the switch diode 42f is turned ON, the inductance devices 42b and 42c are connected in parallel while the inductance devices 42a and 42d are connected in series, and the inductance devices 42a and 42d and the tuning varactor diode 42e set the tuning frequency.
When the television signals of the UHF band are received, the switch diode 42f is turned OFF.
According to the known construction, the input circuit becomes an equivalent circuit shown in FIG. 7 when the television signals of the low range of the VHF band are received, and the inductance device 47a in the UHF tuning circuit 47 is connected in parallel with the inductance devices 42a and 42b in the VHF tuning circuit 42. Since the inductance value of the inductance device 47a in the UHF tuning circuit 47 is sufficiently smaller than the sum of the inductance values of the inductance devices 42a and 42b in the VHF tuning circuit 42, a signal loss increases when the tuning frequency of the VHF tuning circuit 42 deviates to a higher side, inviting the drop of a gain and deterioration of NF (noise factor) particularly when television signals near the lowest frequency are received.
When the television signals of the UHF band are received, the switch diode 42f that is turned OFF has a small capacitance, and the inductance devices 42b and 42c in the VHF tuning circuit 42 and the switch diode 42f constitute a resonance circuit. As this resonance frequency enters the UHF band, adverse influences are exerted on tuning performance of the UHF tuning circuit 47, inviting the drop of the gain and deterioration of NF near the resonance frequency.